1. Field of the Invention
Variable air volume (VAV) heating/cooling systems with means for controlling the flow of return air. VAV systems are generally classified in Class 62--Subclass 186--Automatic control--of external fluid-air controller or damper; and in Class 165--various automatic control subclasses.
2. Description of the Prior Art
VAV systems are a type of all-air system currently used to condition the air supplied to a space (or "zone") in buildings or plants. The design and configurations of such systems are described in the ASHRAE Handbook and Product Directory (1980 Edition)--Systems, Chapter 3.
Generally, these systems include supply ductwork and a "mixed air" plenum chamber in which outside and return air are mixed prior to further conditioning (by heating or cooling coils) and filtering before being discharged into the supply ductwork. One or more supply fans draw conditioned air from the plenum chamber and force the air into the ductwork to the space requiring the conditioned air. Usually, one or more return fans draw the air back from the conditioned space and force it into the mixed air plenum. A portion of the air discharged from the return fan may be exhausted to the outside.
The air flow rate of the outside air which enters the mixed air plenum chamber is typically controlled by volume dampers and by the relative flow rates of the air at the supply and return fan(s). The static pressure within the mixed air plenum chamber must be negative with respect to the outside barometric pressure in order for outside air to flow into the plenum. At design (maximum) flow conditions, relative flow rates of supply and return air usually are such that a minimum flow rate of outside air is assured to enter the mixed air plenum. The prior art consists of many control systems and variations which control the supply and return fan air flow rates so as to maintain a fixed difference between the two. As an example, U.S. Pat. No. 4,011,735 describes a control system which maintains a fixed relationship between supply and return systems. With this control system, and similar control systems, it is possible that outside air flow rates will be reduced below minimum "design" values as the supply and return fan flow rates decrease.
Such systems do not insure that minimum outside air will be drawn into the plenum and mixed with the recirculated "return" air. If the supply and return air flow rates are reduced below, say 50% percent, of design flow requirements, the outside air flow rate could well be reduced below code requirements, as a result of the higher static pressure (the static pressure becoming less negative than outside pressure) in the mixed air plenum chamber.